The low MOI did not reduce the EV71 viral titer. Serum-free medium was used to avoid problems associated with bovine serum, such as lotto- lot inconsistency, potential prion contamination and the difficulty of removing the high serum protein content during downstream purification steps. In this study, two types of EV71 viral particles were produced from a serum-free Vero cell microcarrier bioreactor system and separated and purified using continuous sucrose gradient ultracentrifugation. These results are very similar to those of poliovirus studies in which two different polio viral particles were isolated and identified by sucrose density centrifugation. Two types of poliovirus structures were observed and characterized by electron microscopy and biochemical assays. The D-antigen, like the F-particle in this study, had a high viral RNA content and a full particle structure. In contrast, the C-antigen lacked RNA content and had an empty particle structure like the E-particle found in the current study. When we compared the total protein yield of these EV71 viral particles from the six batches from the bioreactor, the ratio of the E-particle to the F-particle was consistently 7:3. We are currently investigating the parameters of the Vero cell culture that influence the formation of the E-particle. The two EV71 particles had similar icosahedral structures, but their sizes were slightly different, 31�C33 nm and 33�C35 nm for the E-particle and F-particle, respectively. This size difference is most likely due to differences in the composition of the viral protein components and the viral RNA CUR61414 contents as JM6 described above. Generally, the morphogenesis of the Picornaviridae virus begins with the freshly translated P1 polypeptide forming the promoter for selfassembly into a pentamer unit, followed by formation of the empty capsid shell by additional pentamers. The pre-virion and virion formation requires the specific cleavage of the P1 polypeptide. The P1 polypeptide is cleaved into the VP0, VP1 and VP3 proteins by the viral non-structural protein, 3CD protease.